Thermally rearrangeable PIM-polyimides for gas separation membranes

Membrane gas separations require materials with high permeability and good selectivity. For glassy polymers, the gas transport properties depend strongly on the amount and distribution of free volume, which may be enhanced either by engineering the macromolecular backbone to frustrate packing in the solid state or by thermal conversion of a soluble precursor to a more rigid structure of appropriate topology. The first approach gives polymers of intrinsic microporosity (PIMs), while the second approach is used in thermally rearranged (TR) polymers. Recent research has sought to combine these approaches, and here a new range of thermally rearrangeable PIM-polyimides are reported, derived from dianhydrides incorporating a spiro center. Hydroxyl-functionalized polyimides were prepared using two different diamines: 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (bisAPAF) and 4,6-diaminoresorcinol (DAR). Thermal treatment at 450 °C under N2 for 1 h yielded polybenzoxazole (PBO) polymers, which showed increased permeability, compared to the precursor, in membrane gas permeation experiments. A polymer based on DAR (PIM-PBO-3) exhibited a CO2/N2 selectivity of 30 as prepared, higher than the values of 21-23 obtained for polymers derived from bisAPAF with the same dianhydride (PIM-PBO-1). © 2014 American Chemical Society.